DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

and the constant κ is equal to 1 m 1/3 s −1 in SI
units.
manometer A device for measuring pressure
or pressure differences. It consists of a u-shaped
tube, preferably of glass, partially filled with
a liquid (mercury, oil, or water are commonly
used). The difference in height, h, of the liquid
in the two arms of the tube gives the pressure
difference as p =ρgh, where ρ is the fluid
density, and g is the local acceleration of gravity.
If the meter-kilogram-second units are used,
the pressure has units of Pascal (Pa). Pressure
difference may also be indicated by givingh and
the fluid being used, as in “mmHg”, “inches of
Mercury”, “inches of water”.
mantle The terrestrial planets are differentiated
into layers, generally consisting of an outer
crust, interior mantle, and central core. The
mantle is usually the largest (by volume) of these
layers and is composed of minerals that are intermediate
in density between the iron comprising
the core and the lighter materials (such as
feldspars)whichmakeupthecrusts. TheEarth’s
mantle is composed primarily of magnesium,
iron, silicon, and oxygen; the mantles of the
other terrestrial planets are believed to be similar.
Seismology indicates that there are several
transition zones within the mantle, the result of
changes in the densities and other properties of
the component minerals with depth. Seismic
studies of the Earth’s mantle indicate it is composed
of solid rock, but due to the high temperatures
and pressures, the mantle rocks can deform
and flow over thousands to millions of years.
This ductile flow of the mantle rocks is called
mantle convection and is an efficient process by
which heat is transported from the hot interior of
the Earth to the cooler surface. This convection
is believed to be the driving mechanism for the
platetectonicsoperatingontheEarth. Plumesof
hot material, not necessarily associated with the
mantle convection cells, also occur and produce
the hot spots that are responsible for intra-plate
volcanism such as the Hawaiian Islands.
mantle convection On geologic time scales
theEarth’smantlebehavesasafluidduetosolidstate
creep processes. Because of the heat loss
from the interior of the Earth to its surface, the
© 2001 by CRC Press LLC
March 5th event
deep rocks are hot and the shallow rocks are
cool. Because of thermal contraction the cool
near surface rocks are more dense than the hot
deep rocks. This leads to a gravitational instability
with the near surface dense rocks sinking
and the deeper light rocks rising. Plate tectonics,
with the subduction of the cool, dense lithosphere,
is one consequence.
mantletransitionzone Azonethatislocated
between depths of 410 and 660 km where seismic
velocity and density increase markedly with
increasing depth. Sometimes the mantle transition
zone is taken to include the uppermost portion
of the lower mantle down to depths of about
750 to 800 km. Depth distributions of seismic
velocity and density, and their discontinuities
at depths of 410 and 660 km can be explained
by pressure and temperature dependency, and
phase transformations of major composite minerals
(olivine) in pyrolite composition.
March 5th event A gamma ray transient, observed
with 12 spacecraft in 1979, that caused
great controversy. Its properties demonstrated
that it could be a distinct, new class of highenergy
transient. The March 5th event was
clearly identified with the supernova remnant
N49 in the Large Magellanic Cloud (from 55
kiloparsecs away). Its intensity, however, made
it the brightest “gamma ray burst” to date, and
led many to conclude that its apparent N49
source location must be accidental and that it
came instead from a few parsecs away in the
nearby interstellar region, then thought typical
of gamma ray bursts. A few much smaller events
(see soft gamma repeaters) were seen from the
same direction over the years, contrasting to
the apparent lack of repetition of usual gamma
ray bursts, adding to the confusion. Now it is
known, ironically, that gamma ray bursts originate
from cosmological sources, considerably
more distant than even the neighboring galaxies,
and that the March 5th event and the four known
soft gamma repeaters do originate in “magnetar”
galactic and LMC supernova-remnant neutron
stars, including N49. Only one other transient
similar to the March 5th event has been detected
in three decades of space-age monitoring, seen
on August 27, 1998, confirming all aspects of
this interpretation.
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